1. Field of the Invention
The field of technology addressed by this invention is that of hollow ceramic microspheres. These microspheres are useful as lightweight fillers in composites, as fillers for reflective and luminescent coatings, and for thermal insulation and catalyst support applications. Further uses are for the encapsulation of nuclear fusion materials for inertial confinement fusion, for the fabrication of low dielectric constant materials by the introduction of controlled porosity, and for the manufacture of substrates for high-performance multi-chip modules in the microelectronic packaging industry. In particular, this invention relates to methods for the manufacture of microspheres for these and other uses.
2. Description of the Prior Art
One manufacturing method in current commercial use is disclosed by Beck, W.R., et al. U.S. Pat. No. 3,365,315 (Jan. 23, 1968). According to this method, glass particles containing a blowing agent are blown into shells in a gas flame. A second method involves the formation of atomized droplets or powders formed by gelation or spray drying, and the feeding of these droplets through a vertical shell blowing furnace where the water vapor entrapped in the droplets creates internal cavities. This method and variations on it are disclosed by Hendricks, C.D., et al., J. Nucl. Mater. 85/86:107 (1979); Veatch, F., et al. U.S. Pat. No. 3,030,215 (Apr. 17, 1962), Netting, D.I., U.S. Pat. Nos. 3,794,503 (Feb. 26, 1974), 3,796,777 (Mar. 12, 1974), and 3,888,957 (Jun. 10, 1975); and Walsh, R.J., U.S. Pat. No. 3,161,468 (Dec. 15, 1964). Hollow microspheres produced by these methods are, in most cases, limited to glass compositions and are typically hundreds of microns in diameter.
Other methods in the literature which are effective in forming smaller microspheres include: a dual-nozzle hollow drop generation technique as described by Kim, K.K., et al., J. Am. Ceram. Soc. 74(8):1987 (1991); a spray pyrolysis method as described by Jada, S.S., J. Mater. Sci. Lett. 9(5):565 (1990), and Gadalla, M., et al., J. Mater. Res. 5(12):2923 (1990); an emulsion evaporation technique as described by Sarikaya, Y., et al., Ceramic International 14:239 (1988); and a water extraction sol-gel technique as described by Sowman, H.G., U.S. Pat. No. 4,349,456 (Sep. 14, 1982).
The Sowman technique involves the dispersion of an aqueous sol of the microsphere wall material, i.e., the ceramic precursor, in a dehydrating liquid such as a lower alkyl alcohol. The dehydrating agent causes the ceramic precursor to concentrate at the periphery of each droplet in the dispersion, thereby forming a defined spherical shell of gelled ceramic material, with a void at the center of the cell which is filled with a mixture of the dehydrating liquid and water. The liquid is then evaporated from the spherical shells, and the shells are calcined to harden the ceramic and convert the shells to hollow ceramic microspheres.
An advantage of this technique is that it provides chemically homogeneous microspheres and can be used to form microspheres of a wide range of single or multicomponent ceramic compositions. A disadvantage of the technique is that the microspheres which it produces are of a broad size distribution and the process offers little control over particle morphology. These and other disadvantages of the Sowman process and the prior art in general are addressed by the present invention.